// 执行上下文的角度，var/let
function foo() {
  var a = 1;
  let b = 2;
  {
    let b = 3; // 维护一个栈
    var c = 4;
    let d = 5;
    console.log(a);
    console.log(b);
  }
  console.log(b);
  console.log(c);
  console.log(d);//报错 被销毁
}
foo();